Planar defect formation in the γ′ phase during high temperature creep in single crystal CoNi-base superalloys

Abstract

The structure and formation mechanism of extended planar defects in the γ/γ′ microstructure of creep deformed CoNi-base single crystal superalloys have been studied by conventional and advanced transmission electron microscopy (TEM). Planar defects in numerous isolated as well as contiguous γ′ precipitates on {111} planes reveal a characteristic configuration whereby superlattice intrinsic stacking faults (SISF) are fully embedded within antiphase boundaries (APB). Detailed analysis revealed that a leading 1/3[1¯1¯2] superpartial dislocation first creates an SISF. The SISF is then transformed into an APB by a trailing 1/6[1¯1¯2] partial dislocation. The partial is left inside the precipitate and remains as a dislocation loop. Thus, the entire shearing process constitutes a crystallographic slip of type 1/2[1¯1¯2]. A force balance analysis indicates that the initial APB energy exceeds the SISF energy. However, energy-dispersive X-ray spectroscopy (EDXS) indicates pronounced local reordering and diffusion processes near both types of planar defects. The APB qualitatively adopts the composition of the γ phase whereas the SISF locally changes its composition towards that of the Co3W–D019 phase. We propose that these atomic diffusion processes determine the formation and shrinkage of the loops. A post mortem in situ TEM heating experiment shows that with increasing temperature the APBs exhibit complete faceting into {100} planes followed by coarsening, eventually leading to disintegration of the γ′ precipitate. This indicates a detrimental impact of APBs as potential nuclei for fragmentation of the γ/γ′ microstructure in CoNi-base superalloys.